Dyno mystery brain teaser

[Venom Lover]

I haven't dyno'd my Viper since getting smooth tubes, so I figured I'd go do a run today to see what they bought me. The dyno results are shown below. The blue power curve is the curve from a month ago before smooth tubes. The green and red curves are today's runs with smooth tubes, and as you'd expect, they start out above the blue curve. However, around 4700 rpm the curves cross over, and in fact the results show I'm making 431-433 peak rwhp now vs. nearly 450 before. UGH!

So, just for my own amusement and another $25, I put the stock inlet tubes back on (and checked the filters in the air box while I had it off -- they looked fine), and we did another run. The results were nearly identical to the other results from today. Same shaped curves, same crossover with the curves from before at about 4700 rpm. About 2 rwhp less at peak with the stock tubes.

So that made me really confused. We checked for arc-ing on any of the spark plug wires, and nothing. All I can think is that this run was with the Purner slick set-up that I have on the rears, which are I believe about an inch larger overall diameter. The dyno guy said for sure that would make no difference on the power vs. rpm curve, but I have to think that's the difference. The car ran 11.73 sec at 120.35 mph on Sunday at Carlsbad. I can't possibly be down nearly 20 hp, can I???

Any thoughts? I'm going back tomorrow with the street tires on the car to do a re-run. I'll let you know what happens.

(One interesting thing I remember is that somewhere around 4000 rpm an interesting deflection started occurring on the sidewalls of the rear tires above the dyno drum....I will guess that deflection somehow results in a decrease in the torque measured by the dyno and thus affects the power measurement....)

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[Tom Welch]

Your slicks can cause a difference on a dyno run if the pressure of the tires was low. Here is the reason:

The design of the slick has a thin sidewall that "wrinkles" as to soften the hit of the drivetrain and obtain better traction at the drag strip, not to mention the softer compound of the tire which aids in traction.

If your tires were low, they would 1.)need more power to turn them, and 2.) would absorb some of the torque that is supposed to be transfred to the 6000 lb drum that you are trying to turn.

Also if you are on a Dynojet 24B it is designed for high horsepower cars ie; 1000 hp. Running a car rated at less than half of the capacity of that dyno is kind of like weighing an apple on your bathroom scale. Repeatability is not easy, especially from one day to the next.

Air density and temp also play a factor even though the computer of the dyno "corrects" for this. I personally have seen 20 hp swings from one day to the next.

One big question: did you run in 4th gear? I hope so.

Another thought, your power difference of 20 hp is really only a small fraction considering the total output of the car. Something as simple as gasoline quality could make a 2-3% difference in power.

tom

"if we had some ham, we could have some ham and eggs, IF we had some eggs!"

 

[Tom F&L GoR]

If you're using pump gasoline, the use of oxygenates may decrease the BTU energy per gallon by 2.7% (the amount of O2 required to be present via the use of alcohols or ethers) but the ECU should have learned this rather quickly. There isn't enough wiggle room in other pump gas features to account for such a large difference.

What I don't know is whether learned or not, if the open loop fuel rates adjust for this. Also, with a Gen II engine, Sean Roe's work with the VTEC showed at WOT the engine runs rich and ********, so a little leaner shouldn't have hurt you.

Not much help, but a little more info, I hope.

 

[Mike Brunton]

Venom,

I have to disgree with Tom on the effect of the slicks. Yes, it's true that they can wrinkle. On the drag strip, the deformation of the tire "stores" energy which would otherwise be lost to wheelspin. When the tires grip, they have sort of a rubber ball effect where they release that stored energy by returning to their normal shape, sort of 'slingshoting' you away. Slicks tend to baloon upwards (become a bit narrower and higher) at higher speeds, but I certainly don't think you would lose power on the dyno due to wrinkling. If you did, it would not be a constant dip at higher power levels like you saw. The tires can't wrinkle infinitely - the more they wrinkle, the more resistance they have to wrinkling more. At some point, they just spin. If you are getting relatively constant acceleration (which you are), they will wrinkle at a roughly constant amount, negating and effect that would have on your #'s. As for the overall height of the tires, the dynojet doesn't care what your overall driveline gearing is (including tire height), it knows your RPM's and it knows how fast the drum is spinning, which is all it needs. The one thing that COULD have happened is that the tires (being low in pressure) ballooned up a bit and became a bit taller. I'm not sure if the Dynojet samples gearing data constantly or only at the beginning of the run, so if your tire height changed (due to ballooning) then that would show a constant error at higher RPM's where the ballooning started to take place.

Also, your dyno numbers are corrected, so the graph should show the same curve whether you test on a cold day or a hot one, so this isn't an issue of air temperature.

I'd look somewhere else for your answer. I think Tom may be right about the gasoline being the problem. Also was your car heat soaked when you dynoed it? If your car is too cool, you will make less power... if it is too hot, you will make less power. When you're talking >400RWHP, 10-20hp isn't THAT much of a variance, and it's certainly within the scope of reality. Car temps and how heat soaked the motor was can make a BIG difference. The Viper V10 does not like to be heat soaked - it robs quite a bit of power.

If you are really concerned, then hook up a data gathering program and record your motor's stats as you make the run, and you may find your answer there. The fact that it's only occuring at high RPM's makes me think it MAY be a fuel quality issue, or even that your plugs are a bit fouled, or slight clog in your fuel filter or something.

 

[Venom Lover]

I don't know if the mystery is fully solved yet, but on my way back to the dyno shop today, I started getting misfiring/backfiring, and when I pulled the car back into the garage, there was a spark plug wire actually resting on one of the headers off the left (driver's side) cylinder bank. The side of the wire was fairly well scorched. The funny thing was we checked all the spark plug wires at the dyno shop yesterday, and they all looked OK. Certainly none of them was actually in contact with the headers. Anyway, there is no question I'm currently running on 9 cylinders. I don't know if the problem started yesterday, or if yesterday's results were due to something else and this is a coincidence. I hope replacing the sparkplug wire tomorrow solves the problem, otherwise Pomona ain't happening on Saturday, and that would make my sad since it's the last ever street legal event there....Oh well!

I'll let you know what happens tomorrow.

P.S., the Z06 dyno'd today at 336 rwhp/ 337 rwft-lbs (SAE corrected). 340 rwhp/ 341 rwft-lbs actual. That didn't make me particularly happy either, since most Z06's have been dynoing at 340-345 rwhp bone stock. Bad dyno week for Mikey!

 

[Supercharged Vipermad]

I'm surprised no one has mentioned rolling resistance. Slicks have substantially higher rolling resistance than do street tires, and the power lost to rolling resistance will vary roughly linearly with speed. That would account for the dyno graph you got. I'd certainly try the runs again using the same tires (or at least tire type) you used for your initial dyno run.

 

[Venom Lover]

Thanks for the help! Tom Welch confirmed my theory about the effect of the slicks. I think the reduced weight causes the improvements observed <4000 rpm, and at that point the wrinkling of the sidewalls that set in resulted in the need for more power to the wheels to achieve the same torque on the drum, thereby decreasing the measured torque/power. We shall see today if street tires make a difference!

JonB:
The correction factors were small on all 3 occasions, ranging from 0.97 to 1.00. Looking at the "actual" curves confirms this. Temps for runs 10 and 11 (yesterday) were around 74 deg, and temp for run 7 (2 months ago on street tires) was 90 deg. Relative humidities were all +/- 0.1 in Hg of 30.0. Yes, the shop ran a ram air fan in front of the vehicle on both occasions, an the hood was open on both occasions. Good catch, the Purner drag slick/rim setup is lighter than stock, by how much I don't know. I expect that's why the numbers look better up to 4700 rpm.

Tom Welch:
Yes, the pressure was low (at least what I'd consider low). About 14 psi cold. Definitely there was a deflection in the sidewall that started around 4000+ rpm, consistent with what you are saying. The model of Dynojet is 248C, and the guy does a lot of work with F-bodies and Y-bodies, so I assume 248C works pretty well for very low-power cars

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! Yes, all runs were in 4th gear. Since I saw repeatability and results as expected between my first baseline run (stock) and my second run with K&Ns headers and exhaust, I kind of think this third run should have been close....Anyway, we'll see today if you and Purner are right that it's the slicks.

Tom F&L:
I have used Chevron 92 octane since the day I bought the car, so differences due to gas should be minimized. I can't believe there's enough variation in Chevron 92 from one day to the next that it would account for 20 hp.